AASHTO TP 124:2018 Standard Method of Test for Determining the Fracture Potential of Asphalt Mixtures Using the Flexibility Index Test (FIT).
5.2. The calculated FI indicates an asphalt mixture’s overall capacity to resist cracking related damage (Al-Qadi et al. (2015)). Generally, a mixture with higher Fl can resist crack propagation for longer time duration under tensile stress. The Fl should not be directly used in structural design and analysis of pavements. F! values, obtained using this procedure, are used in ranking the cracking resistance of alternative mixtures for a given layer in a structural design. The G, parameter is dependent on specimen size, loading time, and is temperature dependent. Fracture mechanisms for viscoelastic materials are influenced by crack front viscoelasticity and bulk material (far from the crack front) viscoelasticity. Total calculated G, from this test includes the amount of energy dissipated by crack propagation, viscoelastic mechanisms away from the crack front, and other inelastic irreversible processes (frictional and damage processes at the loading support points) (Doll et al., 2016).
5.3. GjIs one of the parameters used to calculate the F!, which is further used to predict AC mixture fracture potential. It also represents the main parameter input in more complex analyses based on a theoretical crack (cohesive zone) model. In order to be used as part of a cohesive zone model, fracture energy as calculated from the experiment shall be corrected to detemine energy associated with crack propagation only. A correction factor may be used to eliminate other sources of inelastic energy contributing to the total fracture energy calculated directly from the experiment.
5.4. This test method and Fl can be used to rank the cracking resistance of asphalt mixtures containing various asphalt binders, modifiers of asphalt binders, aggregate blends, fibers, and recycled materials.
5.5. The specimens can be readily obtained from SGC compacted cylinders or from pavement cores with a diameter of 150 mm.
6.1. Testing Machine—A FIT system consists of a closed-loop axial loading device, a load measuring device, a bend test fixture, specimen deformation measurement devices, and a control and data acquisition system. A constant displacement-rate device, such as a closed loop, feedbackcontrolled servo-hydraulic load frame, shall be used.
Note 1—An electromechanical, screw-driven machine maybe used if results are comparable to a closed loop, feedback-controlled servo-hydraulic load frame.AASHTO TP 124 pdf download.